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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_functiontypes.use_cases"></a><a class="link" href="use_cases.html" title="Use Cases"> Use Cases</a> |
| </h2></div></div></div> |
| <p> |
| Generic libraries that accept callable arguments are common in C++. Accepting |
| a callable argument of builin type often involves a lot of repetitive code |
| because the accepting function is overloaded for different function arities. |
| Further, member functions may have <code class="literal">const</code>/<code class="literal">volatile</code>-qualifiers, |
| a function may take a variable number of (additional, POD-typed) arguments |
| (such as <code class="literal">printf</code>) and several C++ implementations encode |
| a calling convention with each function's type to allow calls across language |
| or (sub-)system boundaries. |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">R</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">accept_function</span><span class="special">(</span><span class="identifier">R</span><span class="special">(*</span> <span class="identifier">func</span><span class="special">)());</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">R</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">accept_function</span><span class="special">(</span><span class="identifier">R</span><span class="special">(&</span> <span class="identifier">func</span><span class="special">)());</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">R</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">C</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">accept_function</span><span class="special">(</span><span class="identifier">R</span><span class="special">(</span><span class="identifier">C</span><span class="special">::*</span> <span class="identifier">func</span><span class="special">)());</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">R</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">C</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">accept_function</span><span class="special">(</span><span class="identifier">R</span><span class="special">(</span><span class="identifier">C</span><span class="special">::*</span> <span class="identifier">func</span><span class="special">)()</span> <span class="keyword">const</span><span class="special">);</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">R</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">C</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">accept_function</span><span class="special">(</span><span class="identifier">R</span><span class="special">(</span><span class="identifier">C</span><span class="special">::*</span> <span class="identifier">func</span><span class="special">)()</span> <span class="keyword">volatile</span><span class="special">);</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">R</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">C</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">accept_function</span><span class="special">(</span><span class="identifier">R</span><span class="special">(</span><span class="identifier">C</span><span class="special">::*</span> <span class="identifier">func</span><span class="special">)()</span> <span class="keyword">const</span> <span class="keyword">volatile</span><span class="special">);</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">R</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">accept_function</span><span class="special">(</span><span class="identifier">R</span><span class="special">(*</span> <span class="identifier">func</span><span class="special">)(...));</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">R</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">accept_function</span><span class="special">(</span><span class="identifier">R</span><span class="special">(&</span> <span class="identifier">func</span><span class="special">)(...));</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">R</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">C</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">accept_function</span><span class="special">(</span><span class="identifier">R</span><span class="special">(</span><span class="identifier">C</span><span class="special">::*</span> <span class="identifier">func</span><span class="special">)(...));</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">R</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">C</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">accept_function</span><span class="special">(</span><span class="identifier">R</span><span class="special">(</span><span class="identifier">C</span><span class="special">::*</span> <span class="identifier">func</span><span class="special">)(...)</span> <span class="keyword">const</span><span class="special">);</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">R</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">C</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">accept_function</span><span class="special">(</span><span class="identifier">R</span><span class="special">(</span><span class="identifier">C</span><span class="special">::*</span> <span class="identifier">func</span><span class="special">)(...)</span> <span class="keyword">volatile</span><span class="special">);</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">R</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">C</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">accept_function</span><span class="special">(</span><span class="identifier">R</span><span class="special">(</span><span class="identifier">C</span><span class="special">::*</span> <span class="identifier">func</span><span class="special">)(...)</span> <span class="keyword">const</span> <span class="keyword">volatile</span><span class="special">);</span> |
| |
| <span class="comment">// ... |
| </span> |
| <span class="comment">// needs to be repeated for every additional function parameter |
| </span><span class="comment">// times the number of possible calling conventions |
| </span></pre> |
| <p> |
| The "overloading approach" obviously does not scale well: There might |
| be several functions that accept callable arguments in one library and client |
| code might end up using several libraries that use this pattern. On the developer |
| side, library developers spend their time solving the same problem, working |
| around the same portability issues, and apply similar optimizations to keep |
| the compilation time down. |
| </p> |
| <p> |
| Using Boost.FunctionTypes it is possible to write a single function template |
| instead: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">F</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">accept_function</span><span class="special">(</span><span class="identifier">F</span> <span class="identifier">f</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="comment">// ... use Boost.FunctionTypes to analyse F |
| </span><span class="special">}</span> |
| </pre> |
| <p> |
| The combination with a tuples library that provides an invoker component, such |
| as <a href="../../../../fusion/index.html" target="_top">Boost.Fusion</a>, allows to |
| build flexible callback facilities that are entirely free of repetitive code |
| as shown by the <a href="../../../../function_types/example/interpreter.hpp" target="_top">interpreter |
| example</a>. |
| </p> |
| <p> |
| When taking the address of an overloaded function or function template, the |
| type of the function must be known from the context the expression is used |
| in. The code below shows three examples for choosing the <code class="literal">float(float)</code> |
| overload of <code class="literal">std::abs</code>. |
| </p> |
| <pre class="programlisting"><span class="keyword">float</span> <span class="special">(*</span><span class="identifier">ptr_absf</span><span class="special">)(</span><span class="keyword">float</span><span class="special">)</span> <span class="special">=</span> <span class="special">&</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">abs</span><span class="special">;</span> |
| |
| |
| <span class="keyword">void</span> <span class="identifier">foo</span><span class="special">(</span><span class="keyword">float</span><span class="special">(*</span><span class="identifier">func</span><span class="special">)(</span><span class="keyword">float</span><span class="special">));</span> |
| |
| <span class="keyword">void</span> <span class="identifier">bar</span><span class="special">()</span> |
| <span class="special">{</span> |
| <span class="identifier">foo</span><span class="special">(&</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">abs</span><span class="special">);</span> |
| <span class="special">}</span> |
| |
| |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">b</span><span class="special">,</span> <span class="identifier">e</span><span class="special">,</span> <span class="identifier">o</span><span class="special">,</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="keyword">float</span><span class="special">(*)(</span><span class="keyword">float</span><span class="special">)>(&</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">abs</span><span class="special">));</span> |
| </pre> |
| <p> |
| The library's type synthesis capabilities can be used to automate overload |
| selection and instantiation of function templates. Given an overloaded function |
| template |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">R</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T0</span><span class="special">></span> |
| <span class="identifier">R</span> <span class="identifier">overloaded</span><span class="special">(</span><span class="identifier">T0</span><span class="special">);</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">R</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T1</span><span class="special">></span> |
| <span class="identifier">R</span> <span class="identifier">overloaded</span><span class="special">(</span><span class="identifier">T0</span><span class="special">,</span><span class="identifier">T1</span><span class="special">);</span> |
| |
| <span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">R</span><span class="special">.</span> <span class="keyword">typename</span> <span class="identifier">T0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T1</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T2</span><span class="special">></span> |
| <span class="identifier">R</span> <span class="identifier">overloaded</span><span class="special">(</span><span class="identifier">T0</span><span class="special">,</span><span class="identifier">T1</span><span class="special">,</span><span class="identifier">T2</span><span class="special">);</span> |
| </pre> |
| <p> |
| we can pick any of the three overloads and instantiate the template with template |
| arguments from a type sequence in a single expression: |
| </p> |
| <pre class="programlisting"><span class="keyword">static_cast</span><span class="special"><</span><a class="link" href="reference/synthesis.html#boost_functiontypes.reference.synthesis.function_pointer" title="function_pointer">function_pointer</a><span class="special"><</span><span class="identifier">Seq</span><span class="special">>::</span><span class="identifier">type</span><span class="special">>(&</span> <span class="identifier">overloaded</span><span class="special">)</span> |
| </pre> |
| <p> |
| This technique can be occasionally more flexible than template argument deduction |
| from a function call because the exact types from the sequence are used to |
| specialize the template (including possibly cv-qualified reference types and |
| the result type). It is applied twice in the <a href="../../../../function_types/example/interface.hpp" target="_top">interface |
| example</a>. |
| </p> |
| <p> |
| Another interersting property of callable, builtin types is that they can be |
| valid types for non-type template parameters. This way, a function can be pinpointed |
| at compile time, allowing the compiler to eliminate the call by inlining. The |
| <a href="../../../../function_types/example/fast_mem_fn.hpp" target="_top">fast_mem_fn example</a> |
| exploits this characteristic and implements a potentially inlining version |
| of <a href="../../../../bind/mem_fn.html" target="_top">boost::mem_fn</a> limited to |
| member functions that are known at compile time. |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2004 -2007 Tobias Schwinger<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) |
| </p> |
| </div></td> |
| </tr></table> |
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